1. Field of the Invention
The present invention relates to an exhaust system for a small-sized boat. More particularly, the present invention relates to an exhaust system for a small-sized boat that reduces low-frequency exhaust sounds.
2. Description of the Background Art
A conventional exhaust system for a small-sized boat includes a water muffler at one point along exhaust pipes extending from an engine disposed in the boat body, and the water muffler includes a plurality of expansion chambers (see Japanese Patent Laid-Open No. Heisei 10-212936 (Abstract, Paragraph 0027, FIG. 1 and FIG. 3), for example).
In addition, there is another known exhaust system that includes two water mufflers (see Japanese Patent Laid-Open No. Heisei 8-119196 (Abstract, Paragraph 0012, FIG. 2), for example).
The above-described conventional exhaust systems are considered to be suitable for reducing exhaust noises in a relatively high frequency band, since the water mufflers thereof include a plurality of expansion chambers.
However, exhaust noises in a region of about 3000 to 7000 rpm of engine revolutions (for example, in a case of a four-cycle four-cylinder engine, a region of about 100 to 200 Hz) are in fact disturbing noises in a small-sized boat. Moreover, since a small-sized boat discharges cooling water into exhaust gas (see the foregoing discussed Japanese Patent Documents), exhaust gas temperature becomes 100° C. or below, and it has become clear that high frequency noises are not generated under such a condition.
In other words, there have been problems, with respect to the conventional exhaust systems for small-sized boats, that they include a plurality of expansion chambers which are actually not necessary, and also that they cannot effectively reduce exhaust noises in a low frequency band, which are most disturbing noises in small-sized boats.
Furthermore, when there are two mufflers, one of the mufflers can reduce low frequency waves; however, there have been problems that the capacity thereof is doubled, that the layout in a narrow space of the boat is difficult, and that cost thereof is also doubled.